The amount of emissions, such as NOx, discharged from an internal combustion engine such as a spark-ignition internal combustion engine or a diesel engine has a strong correlation with the flame temperature (combustion temperature) after ignition. Therefore, controlling the flame temperature to a predetermined temperature effectively reduces the amount of emissions, such as NOx. In general, since flame temperature cannot be detected directly, the flame temperature must be estimated so as to be controlled to the predetermined temperature. Meanwhile, the flame temperature changes with the temperature of a gas mixture before being ignited (hereinafter, may be simply referred to as “gas mixture temperature”). Accordingly, estimating the gas mixture temperature is effective for estimation of the flame temperature.
In particular, in the case of a diesel engine in which air-fuel mixture starts combustion by means of self ignition caused by compression, the ignition timing must be properly controlled in accordance with the operation state of the engine. The ignition timing greatly depends on the gas mixture temperature before ignition. Accordingly, estimating the gas mixture temperature is also necessary for proper control of the ignition timing.
In view of the above, a fuel injection apparatus for a diesel engine disclosed in Japanese Patent Application Laid-Open (kokai) No. 2001-254645 sets a target ignition timing in accordance with the operation state of an engine, and estimates the gas mixture temperature as measured at the target ignition timing on the basis of various operational state quantities which affect the gas mixture temperature, such as engine coolant temperature, intake air temperature, and intake pressure. Subsequently, the apparatus controls the manner of injection (e.g., injection timing and/or injection pressure) of fuel in such a manner that the estimated gas mixture temperature attains a predetermined temperature, to thereby control the ignition timing to coincide with the target ignition timing.
Incidentally, depending on the operation state of an engine, a gas mixture which is produced through mixing of fuel injected into a combustion chamber and a cylinder interior gas is often ignited after the gas mixture reaches the inner wall surface of the combustion chamber. In such case, the gas mixture can be considered (assumed) to stagnate in a generally annular configuration in the vicinity of the side wall (having a generally cylindrical inner wall surface) of the combustion chamber after having reached the inner wall surface of the combustion chamber and at least until ignition of the gas mixture. During such a period in which the gas mixture is stagnant, the temperature of the gas mixture is affected by heat transfer between the gas mixture, and the combustion chamber wall and the like existing around the gas mixture.
However, the above-described conventional apparatus estimates such a gas mixture temperature without consideration of the influence of the above-described heat transfer. Therefore, the estimated gas mixture temperature involves an error, and as a result the conventional apparatus cannot render the ignition timing coincident with the target ignition timing.